Update a Paddle subscription's plan from Next.js

When to use this skill

Use this skill when building "Upgrade plan," "Switch tier," or "Change subscription" actions in your authenticated user's billing UI. It covers a Next.js 15 (App Router) Server Action that calls paddle.subscriptions.update(), the four prorationBillingMode choices and when to pick each, the replace not append semantics of the items array, and the security checks every plan-change action needs.

This is the initiating side of subscription changes. Pair it with:

• subscription-sync — your webhook handler will get a subscription.updated event after the change. Your DB mirror picks up the new plan, status, and pricing.
• subscription-cancel — same auth/ownership shape; if you're building both, share the helpers.

Prerequisites

• A Paddle account with at least two prices the user can switch between (e.g. monthly Starter and monthly Pro). Sandbox is fine.
• Server-side PADDLE_API_KEY available — Server Action only.
• Customer + subscription state mirrored from webhooks (see subscription-sync).
• An auth system. Examples use Supabase.

NEXT_PUBLIC_PADDLE_ENV=sandbox             # or "production"
PADDLE_API_KEY=pdl_sdbx_apikey_...

NEXT_PUBLIC_SUPABASE_URL=...
NEXT_PUBLIC_SUPABASE_ANON_KEY=...
SUPABASE_SERVICE_ROLE_KEY=...              # or SUPABASE_SECRET_KEY (new opaque sb_secret_*)

Choose your prorationBillingMode

This is the one parameter most people get wrong. Paddle's subscriptions.update accepts five values; for a typical "Upgrade to Pro" flow the right choice is prorated_immediately.

There is no default. You must pass one of the five values.

Upgrade, downgrade, or term change?

subscriptions.update is one endpoint covering several distinct user-visible flows. The right prorationBillingMode depends on which:

Same-billing-interval rule for items

A subscription's items must all share the same billing interval. So if the user has a Pro monthly plan plus a monthly addon and switches to Pro annual, the addon must also become annual at the same time — it cannot stay monthly. Practically:

• If you sell tier + addons, your items array on the update must include the new addon price IDs that match the new term.
• If you sell tier-only, this doesn't affect you.

This is a constraint on what you submit to the SDK, not just a UX consideration. The API rejects mixed-interval items arrays.

Preview before committing

For non-trivial flows — especially term changes, anything where the prorated amount isn't obvious, or anything you're going to ask the user to confirm — show the preview before committing. Paddle's subscriptions.previewUpdate takes the same request body as update but doesn't apply anything:

const preview = await paddle.subscriptions.previewUpdate(subscriptionId, {
  items: [{ priceId: newPriceId, quantity: 1 }],
  prorationBillingMode: "prorated_immediately",
});

// preview.immediateTransaction       — the prorated charge details (if any)
// preview.recurringTransactionDetails — the new ongoing price
// preview.nextBilledAt                — when the next renewal will hit

Render those numbers on a confirm screen ("You'll be charged $X today, then $Y/year starting <date>"). When the user confirms, run the same body through subscriptions.update. Don't try to compute the prorated amount yourself — the preview is the source of truth, and the math accounts for tax, currency, and existing credit balances you may not be aware of.

For simple "Upgrade to Pro" flows where the price difference is small and obvious, you can skip the preview and trust the user to know what they're agreeing to from the button label. For everything else, the preview pays for itself in support tickets you don't get.

How paddle.subscriptions.update handles items

The items field on the update is a full replacement of the subscription's line items, not an append:

// BEFORE: subscription has items: [{ priceId: 'pri_starter_monthly', quantity: 1 }]

await paddle.subscriptions.update(subscriptionId, {
  items: [{ priceId: "pri_pro_monthly", quantity: 1 }],
  prorationBillingMode: "prorated_immediately",
});

// AFTER: subscription has items: [{ priceId: 'pri_pro_monthly', quantity: 1 }]
//        the Starter item is gone — replaced by Pro.

For a single-item plan change, that's exactly what you want. Do not fetch the existing items and concat the new one — you'd end up with a subscription that's billed for both plans simultaneously.

For multi-item subscriptions (a base plan + addons), you do need to think about which items you're keeping. In that case, fetch the current subscription, modify the items array carefully, and pass the modified array back.

The full Server Action

// src/actions/subscription.ts
"use server";

import { revalidatePath } from "next/cache";
import { getPaddleInstance } from "@/utils/paddle/get-paddle-instance";
import { createServerInternalClient } from "@/utils/supabase/server-internal";
import { createServerClient } from "@/utils/supabase/server";

export async function updateSubscription(subscriptionId: string, newPriceId: string) {
  // 1. Authenticate. Reject anonymous requests before any DB query or SDK call.
  const supabase = await createServerClient();
  const {
    data: { user },
  } = await supabase.auth.getUser();
  if (!user || !user.email) {
    return { error: "Not authenticated" };
  }

  // 2. Verify the authenticated user owns this subscriptionId. Same email-bridge
  //    pattern as subscription-cancel.
  const internal = createServerInternalClient();

  const { data: customerRow } = await internal
    .from("customers")
    .select("customer_id")
    .eq("email", user.email)
    .single();
  if (!customerRow) return { error: "No Paddle customer" };

  const { data: subRow } = await internal
    .from("subscriptions")
    .select("customer_id")
    .eq("subscription_id", subscriptionId)
    .single();
  if (!subRow || subRow.customer_id !== customerRow.customer_id) {
    return { error: "Forbidden" };
  }

  // 3. Apply the plan change. Replace items with the single new item; bill the
  //    prorated difference now (the right call for an "Upgrade" button). The
  //    default behaviour on payment failure is `prevent_change` — the upgrade
  //    only applies if the prorated charge succeeds. That's the safe choice for
  //    self-serve. See "Handling payment failure" below for when 'apply_change'
  //    is the right override.
  const paddle = getPaddleInstance();
  const subscription = await paddle.subscriptions.update(subscriptionId, {
    items: [{ priceId: newPriceId, quantity: 1 }],
    prorationBillingMode: "prorated_immediately",
  });

  // 4. Refresh cached UI.
  revalidatePath("/dashboard/subscriptions");

  // 5. Slim DTO — don't leak the raw Subscription.
  return {
    success: true,
    priceId: subscription.items[0]?.price?.id ?? null,
    status: subscription.status,
  };
}

What the user sees vs what gets billed

A prorated_immediately upgrade triggers Paddle to:

1. Generate a transaction for the prorated difference between the old and new plan, billed against the customer's existing payment method.
2. Send a transaction.created (then transaction.completed if the charge succeeds, or transaction.payment_failed if it doesn't).
3. Send a subscription.updated reflecting the new items and next_billed_at recalculated for the new plan.

If the prorated charge fails (declined card, expired method), what happens depends on onPaymentFailure — see the next section.

Handling payment failure

When you pick an immediate proration mode (prorated_immediately or full_immediately), Paddle attempts to charge the customer synchronously during the update call. If that charge fails, the onPaymentFailure field decides whether the plan change still applies:

Pick based on the flow:

• Default prevent_change is appropriate for most self-serve UX: the user clicks Upgrade, the card declines, they see "Payment failed" and can update their card and retry. Every step is explicit. You don't need to set it explicitly — leaving it out gives you this behaviour.
• apply_change is appropriate for admin-driven flows (a CS rep upgrading a customer mid-trial) or for "switch tier first, collect later" patterns. The customer immediately gets the new plan; you collect the past-due transaction asynchronously. Don't pick this without thinking about your collections process — you've effectively extended credit.

For the admin-driven case, set onPaymentFailure explicitly:

// Admin-driven upgrade: apply the change even if the card fails. The customer
// gets the new plan; the prorated charge sits in past_due until you collect.
await paddle.subscriptions.update(subscriptionId, {
  items: [{ priceId: newPriceId, quantity: 1 }],
  prorationBillingMode: "prorated_immediately",
  onPaymentFailure: "apply_change",
});

The Node SDK uses camelCase (onPaymentFailure); the wire format is on_payment_failure.

Common pitfalls

• Wrong prorationBillingMode. The defaults table is the heart of this skill. do_not_bill for a self-serve upgrade is a billing-accounting hole; full_immediately overcharges; prorated_next_billing_period surprises users who expected a charge. For "Upgrade to Pro" UX, always prorated_immediately.
• Appending instead of replacing items. Pulling subscription.items from the mirror, concatenating the new item, and sending the combined array results in a subscription billed for two plans. The endpoint replaces — pass only what you want.
• No ownership check. Same as subscription-cancel: trusting the subscriptionId input lets any authenticated user upgrade any subscription's plan.
• No auth check. Server Actions are callable from anywhere — anonymous, scripted, or via direct POST. Always verify the session.
• Optimistic UI based on the action's return. The action returning { success: true } means Paddle accepted the change request. It does NOT mean the prorated charge succeeded. Wait for transaction.completed (via webhook) before granting features that depend on payment success.
• Missing revalidatePath. UI shows the old plan until manual refresh.
• Returning the raw Subscription. Same as cancel — the SDK's instance is large; slim it.
• Skipping the preview step on a non-trivial change. For term changes especially, the prorated calculation isn't obvious — Paddle may credit existing balances, apply tax, or convert currency in ways your back-of-envelope estimate misses. Showing the user a confirm screen with the actual numbers from previewUpdate before they commit avoids "wait, why was I charged this much?" tickets. Skip the preview only when the price impact is small and obvious.
• Not knowing onPaymentFailure exists. The default (prevent_change) is correct for self-serve and you don't need to override it. The trap is not realizing the option exists when you have a flow where you do need apply_change — e.g. a CS rep upgrading a customer with a temporarily-failing card. If you don't know the field is there, you'll work around the limitation in clumsier ways.
• Mixed billing intervals on items. If you sell tier + addons and let users switch between monthly and annual, the addons must move with the tier. Submitting [{ priceId: 'pri_pro_annual' }, { priceId: 'pri_addon_monthly' }] will fail — Paddle rejects mixed-interval items arrays. When designing your "switch term" flow, plan for swapping addon price IDs alongside the tier.
• Treating downgrade like upgrade. prorated_immediately on a Pro → Starter switch issues a prorated refund mid-period — most apps don't want that for a "downgrade" UX. Use prorated_next_billing_period so the change applies at renewal without refunding.

Verify the integration

1. Have a sandbox subscription on the cheaper of two plans (e.g. Starter monthly).
2. Call paddle.subscriptions.previewUpdate with the upgrade body. Confirm the returned numbers (immediate transaction, recurring details, next-billed-at) match what you'd expect.
3. Click "Upgrade to Pro." The action should return { success: true, priceId: 'pri_pro_monthly', status: 'active' }.
4. In the Paddle dashboard:
   • Subscription's items are now Pro only (Starter is gone).
   • A new transaction with the prorated difference appears, and matches the preview from step 2.
5. Check your DB mirror after the subscription.updated webhook fires: the row has the new price_id and product_id.
6. Switch your sandbox card to a test card that declines and retry the upgrade. With onPaymentFailure: 'prevent_change', confirm the subscription stays on the old plan and your action returns an error. Switch to onPaymentFailure: 'apply_change' and confirm the subscription flips to the new plan and a past_due transaction appears.
7. Try the action with a newPriceId you don't actually offer. Confirm Paddle rejects it (or — better — your action validates the newPriceId against your PricingTier constant before calling the SDK).
8. Try the action while logged out, or with a forged subscriptionId belonging to a different user. Confirm both are rejected before the SDK is called.

Related docs

• Upgrade or downgrade a subscription
• Proration billing modes
• Update a subscription - API
• Preview an update to a subscription - API